FOA Guide to fiber optics 


Topic: Fiber Optic Loss Measurements

Table of Contents: The FOA Reference Guide To Fiber Optics



Understanding The Math Of Insertion Loss Testing

    In order to test “insertion loss” or the direct loss of a fiber optic cable or cable plant using a light source and power meter (LSPM in most international standards or optical loss test set – OLTS – in many articles), one must make an initial measurement to determine the “0 dB” reference point with the LSPM and reference cables to be used in making the measurement.
    It has become apparent in technical and standards meetings that what these measurements actually measure are not well understood, even by experts, because of the casual way the reference and test measurements are  often described. The purpose of this paper is to use simple math is to clear up this issue.

Testing the cable plant
    Standard test methods, use a light source [LS] and reference “launch” cable on one end of the “cable to test” and a “receive” reference cable connected to a power meter [PM] on the far end. The test is intended to measure the loss of the connections of the connectors on either end to the reference test cables and the loss of the rest of the cable (which may include splices or additional connections in addition to the fiber.)

OFSTP-14/7: [LS]------ref cable------[A]------cable to test------[B]------ref cable------[PM]


Three ways to set the reference
    All standards offer three methods of setting a  “0 dB” reference using one two or three reference cables. It’s assumed that all reference cables are short enough (generally less than 2m or 6 feet) that the loss of the fiber in them is ignorable and that the connectors are high quality – that is to say low loss. It’s also assumed that the power meter has a large enough detector that it gathers all the light from the end of a fiber so it has a consistent connection to the fiber being connected to its interface.

1 Cable Reference : [LS]-------ref cable-------[PM]

2 Cable Reference: [LS]-------ref cable-------[1]-------ref cable-------[PM]

3 Cable Reference: [LS]-------ref cable-------[1]-------ref cable-------[2]-------ref cable-------[PM]

    Why do we have three different methods? Mostly we believe it came about from three issues: 1) Test equipment with fixed connectors that do not match the connectors on the cable to test cannot be used for the 1 cable reference, 2) Techs working at either end of a length of cable and set the reference with a local light source and power meter and 3) Belief that since you need a launch and receive reference cable, you should make the reference setting using both of them.
    The 3 cable reference has become popular with plug/jack connectors like the MT-RJ or MTP which are generally incompatible with test instruments, but LC connectors may require a similar approach if the power meter or OLTS does not have adapters that allow connecting different types of connectors.
    If you look at the schematic drawings of the three methods, you can see that when you set the reference, one or two connections are included. That means that the loss of those connections are included in setting the 0 dB reference.
    This has led to the misunderstanding that the 3 cable reference , which includes two connections when the reference is set, does not measure the loss of the connectors on the ends of the cable because two connections were included in setting the  0 dB reference and the 1 cable reference (the old Method B) only measures one connector because one is included in setting the reference.
    Unfortunately, it’s not that simple - that's an improper generalization. Any version of the test measures all the connector losses in the cable under test, but subtracts the loss of connections included when setting the “0 dB reference.” Let’s do the math for each method and you will see what we mean.

The 1 Cable Reference, Formerly Method B

Set reference by Method B:

[LS]-------ref cable-------0-[PM]

 where “0” equals the output of the reference cable measured by [PM].

    No connections are included in the reference test (see next paragraph), so when we do the test, we are connecting the connectors of the ref cables directly to the connections on the cable to test and that loss will be included in the measurement.

    We consider the connection to the meter to have "no loss" which may be true with a power meter with a large area detector capturing ligth directly from the end of a connected optical fiber cable. However, many OLTS, especially those designed to make bidirectional tests, may have that connection be fiber-to-fiber and may even have a couple internal to the OLTS. That test set has a double uncertainty, the connection to the fiber under test and the variability of the coupler due to mode power distribution in MM fiber.

OFSTP-14/7: [LS]------ref cable------[A]------cable to test------[B]------ref cable------[PM]

    Thus, with the power meter, we measure loss “L”, we measure connection loss [A], the loss of the fiber and any intermediate connections or splices in the cable to test [CTT] and connection [B] or

L = 0 – ([A]+CTT+[B]) = loss of the cable


The 2 Cable Reference, Formerly Method A

Set the reference with method A but do not change the output of the LS.

2 cable reference : [LS]-------ref cable------0-[1]-------ref cable-------(0-[1])[PM]

 where “0” equals the output of the launch reference cable measured by [PM] as in the 1 cable reference, decreased by the loss of the connection [1] between the reference cables, so the [PM] now measures (0-[1]).

OFSTP-14/7: [LS]------ref cable------[A]------cable to test------[B]------ref cable------[PM]

    Thus, with the power meter, we measure loss “L”, we measure connection loss [A], the loss of the fiber and any intermediate connections or splices in the cable to test [CTT] and connection [B] but our “0 dB reference”  is now “0-[1]” and the loss measured is

L = (0- [1]) – ([A]+CTT+[B]) = measured loss of the cable

    Note the measured loss is the same as the 1 cable reference decreased by the loss of the connection between the reference cables [1] when the “0 dB reference” was set, which is not necessarily the same as saying the loss measured does not include one of the connectors, since the loss of [1] is not necessarily the same as [A] or [B]. That unknown factor causes this method to be more uncertain (many would say "less accurate") than the 1 cable reference.


Is it possible to compensate for the included connections?

    Someone making this test who understands this issue could compensate for the difference by calibrating [1] this way:

Set reference by  1 cable reference :

[LS]-------ref cable-------0-[PM]

Add the receive reference cable and measure again:

2 cable reference : [LS]-------ref cable-----0-[1]-------ref cable------[1][PM]

    This test gives you a direct measurement of [1] which can now be added back in when testing the cable, giving you the same result as method B. But in order to do so requires the ability of the instruments to measure by the 1 cable reference , so it just adds another step to the process – why not just use  1 cable reference in the first place. And the loss measured [1] will be slightly different each time the two connectors are mated, adding to the uncertainty of the measurement.


The 3 Cable Reference, Formerly Method C

    Set the reference with method A but do not change the output of the LS.

Method C: [LS]-----ref cable----0-[1]------ref cable------[2]------ref cable------(0-[1]-[2])[PM]

 where “0” equals the output of the launch reference cable measured by [PM] as in the 1 cable reference, decreased by the loss of the connections [1] and [2] between the reference cables, so the [PM] now measures (0-[1]-[2]).

OFSTP-14/7: [LS]------ref cable------[A]------cable to test------[B]------ref cable------[PM]

    Now with the power meter, when we measure loss “L”, we measure connection loss [A], the loss of the fiber and any intermediate connections or splices in the cable to test [CTT] and connection [B] but our “0 dB reference”  is now “0-[1]-[2]” and the loss measured is

L = (0- [1]-[2]) – ([A]+CTT+[B]) = measured loss of the cable

    The measured loss now is the same as 1 cable reference decreased by the loss of the connection between the reference cables [1] and [2] when the “0 dB reference” was set, which is not necessarily the same as saying the loss measured does not include one of the connectors, since having two unknown connectors causes this method to be more uncertain (many would say "less accurate") than either of the other methods.

Putting some numbers on this methodology
    Consider that the test is being made by someone who is careful with all the procedures and has very good reference cables, which should have mating losses of 0.2 dB or better. If we test a cable plant with the 1 cable reference at 3.0 dB, with the 2 cable reference it will measure 2.8 dB and with the 3 cable reference, 2.6 dB. 
    We’ve done actual tests like this under strictly controlled conditions and got 2.96 dB,  2.66 dB and 2.48 dB. See the table below.
    The measurement uncertainty, called “standard deviation” in statistics, can be determined by making multiple measurements, averaging and calculating standard deviation. What was found is this test here was 1 cable reference had a standard deviation of 0.02 dB, 2 cable reference had a standard deviation of 0.20 dB and 3 cable reference had a standard deviation of 0.24 dB. This is expected because the  1 cable reference has no variation caused by connections included in setting the reference, while the 2 cable reference has one and  3 cable reference has two and every fiber optic connection has some variation each time it is connected.

Test Method   Results
Loss in dB, Standard Deviation
1 Cable Reference 2.96 dB, +/-0.02 dB
2 Cable Reference 2.66 dB, +/-0.20 dB
3 Cable Reference 2.48 dB, +/-0.24 dB


    Another unknown creates a problem with these tests on multimode cable – mode power distribution (MPD.) Even if we control the MPD in the launch cable using a source of known MPD and a mandrel wrap as called for in some standards, as soon as we add another connector, we change the MPD in a fairly uncontrolled fashion. Connectors act as mode modifiers, stripping off higher order modes which are the most lossy in connections but converting low order modes to higher order modes also. So setting the reference with connections adds to the uncertainty of mode power distribution in the testing.
    Yet another issue is the cleanliness of connectors. If one of the connectors is dirty when the reference is set by the 2 or 3 cable reference methods, but the connectors are subsequently cleaned or the dirt falls off, the loss will change because of the extra loss included in the reference setting process. This has even been known to cause a LSPM test to show a “gain” which sounds impossible.

“Fudging” the Methodology
    Manufacturers of test equipment with fixed connector interfaces have invented some creative methods of "fudging" a 1 cable reference test. Some of them do a 2 cable reference, add a hybrid adapter cable or two to allow connection of the cable to test, then do some behind the scenes software calculations and “kazam!” a claim it is a 1 cable reference test because it includes 2 more connections in the test than in the reference! Looking at the info above regarding the uncertainty of measurements anyway, think about how uncertain a measurement is when you try to guess at the loss of several newly added connections!

"0" dB Reference: [LS]-------ref cable------0-[1]-------ref cable-------(0-[1])[PM]

Test: [LS]------ref cable------[X]--adapter---[A]------cable to test------[B]------ref cable------[PM]

L = (0- [1]) – ([X]+[A]+CTT+[B]) = measured loss of the cable

The uncertainty of the measurement is primarily determined by the difference in the losses [1] and [X].

Which method to use?
    Most cabling standards today recognize the need for the 3 cable reference for some types of connectors but prefer the 1 cable reference. Network standards for link loss have referenced all methods, so it's important to check the network specifications. Many telcos still use the 2 cable reference. Whatever method you use, the loss data you document must include the method used to be valid and be comparable to other tests.







 


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